Is concealing the intention to go for Hajj and preparing for it secretly, without informing friends, considered permissible in Islam, especially since this might upset friends who wished to go for Hajj and who might see in this a failure to apply the Hadith "None of you truly believes until he wishes for his brother what he wishes for himself"?

It would have been better for your friend to inform you so that anyone who wished could participate in this good deed. However, he might be excused for keeping it secret, due to the Prophet's (peace be upon him) saying: "Seek help for the fulfillment of your needs by keeping them secret, for every possessor of a blessing is envied." And find excuses for your brethren, and do not let Satan sow discord among you. If they owe you debts and Hajj would affect their repayment, then they must seek your permission.